Genomic Insights Reveal CmNRAMP3 as a Key Regulator of Chrysanthemum Rooting

Chrysanthemum (Chrysanthemum morifolium) is a globally important ornamental crop propagated primarily through stem cuttings. Successful commercial production depends on rapid root initiation and robust root system development. However, certain cultivars exhibit weak rooting competence, delayed establishment, and higher post-transplant mortality, leading to economic losses. Rooting ability is a complex quantitative trait influenced by hormone signaling, stress responses, nutrient metabolism, and polygenic regulation. Although genes such as CmANR1 and CmBT1 have been reported, the broader genetic basis of rooting variation across diverse germplasm remains unclear. Due to these challenges, in-depth genomic research is urgently needed to unravel the molecular mechanisms governing chrysanthemum rooting capacity.

In a study published (DOI: 10.1093/hr/uhaf289) in Horticulture Research (Oxford University Press, 2025), researchers from Nanjing Agricultural University and collaborating institutions conducted a large-scale genomic analysis of rooting traits in chrysanthemum. Evaluating 11 root-related characteristics across 188 accessions over two years, the team combined population genomics, genome-wide association studies (GWAS), transcriptome sequencing, and weighted gene co-expression network analysis (WGCNA). Their work identified 71 significant SNPs and multiple candidate genes associated with key root traits, ultimately pinpointing CmNRAMP3 as a critical regulator influencing rooting performance.

Using high-resolution genotyping-by-sequencing data, the researchers first characterized phenotypic variation in root architecture. Spray cut chrysanthemums (SCC) displayed significantly stronger rooting ability than traditional and wild types, suggesting directional selection during breeding. Selective sweep analysis identified 534 genomic regions potentially shaped by artificial selection for rooting traits. GWAS performed on four core traits-total root length (TL), root surface area (SA), average root diameter (AD), and number of roots (NR)-revealed 71 significant SNPs and 98 candidate genes. Approximately 21% of these candidates were differentially expressed between strong- and weak-rooting genotypes. Hormone-related genes involved in auxin, abscisic acid, and ethylene pathways were prominently enriched, highlighting hormonal crosstalk as central to adventitious root formation. A standout discovery was CmNRAMP3, located within a domestication sweep and associated with root surface area. Expression analysis showed higher levels of CmNRAMP3 in weak-rooting genotypes. Transient overexpression experiments confirmed its inhibitory function: plants overexpressing CmNRAMP3 exhibited significantly reduced TL, SA, AD, and NR. This functional validation establishes CmNRAMP3 as a negative regulator of root system development, providing rare causal evidence in an ornamental polyploid crop with a complex genome exceeding 8 Gb.

"Rooting ability directly determines propagation efficiency and plant vigor in commercial chrysanthemum production," said the study's corresponding author. "By integrating population genomics with functional validation, we moved beyond association to demonstrate causality. Identifying CmNRAMP3 as a negative regulator not only clarifies how rooting capacity evolved during breeding, but also provides a tangible molecular target for improving clonal propagation. This is a significant step toward precision breeding in complex ornamental crops."

The discovery of genetic markers and causal regulators underlying rooting ability opens new avenues for molecular breeding in chrysanthemum. SNP markers linked to favorable haplotypes can accelerate marker-assisted selection for high-rooting cultivars, reducing propagation losses and enhancing uniformity in large-scale production. Beyond chrysanthemums, this integrative framework offers a model for improving rooting performance in other vegetatively propagated crops, including ornamental, horticultural, and forestry species. As global demand for high-quality ornamental plants rises, genomics-guided breeding strategies such as these will become essential tools for sustainable and efficient production systems.